1. Technical Field
The present invention relates to a surface acoustic wave device using an upper limit mode of a stop band of a Rayleigh surface acoustic wave and a surface acoustic wave oscillator including the surface acoustic wave device.
2. Related Art
An interdigital transducer (IDT) and a reflector that constitute a surface acoustic wave element include a plurality of conductive strips. The conductor strips have a periodic structure that provides a frequency band, i.e., a stop band, to reflect a surface acoustic wave (SAW) of a specific frequency domain by a high reflection coefficient.
A surface acoustic wave device made of an ST-cut quartz substrate with a predetermined in-plane rotated angle ψ and enabling an excitation of an upper limit mode of a stop band of Rayleigh surface acoustic waves is known for having an excellent frequency temperature characteristic. Further, a single type IDT only having two electrode fingers in a wavelength of surface acoustic waves can be employed, as it is known, reducing difficulty in miniaturizing electrodes when a frequency is increased and facilitating the increase of the frequency compared to a case of employing a reflection/inversion type IDT electrode (including three electrode fingers in a wavelength of surface acoustic waves) that is required to excite an upper limit of a stop band in related art. Furthermore, it is also known that an excitation of an upper limit mode of a stop band has a smaller frequency variation when a thickness of an electrode film is increased so as to reduce a resistance value of an IDT, i.e., a frequency variation caused by a variation of the thickness of the electrode film, compared to an excitation of a lower limit mode (refer to JP-A-2006-148622 that is a first example of related art).
The first example of related art discloses that a SAW of the upper limit mode of the stop band becomes excitable when a propagation direction of the SAW is shifted from an X axis of quartz crystal.
In JP-A-11-214958 that is a second example of related art, it is described that a stop band is formed by a periodic reflection of a SAW caused by a periodic configuration in which a number of electrode fingers are aligned, and further a reflection/inversion type IDT is also disclosed in detail. Furthermore, according to the description, frequencies of a lower end (lower limit) and an upper end (upper limit) in the stop band are in a resonated state, generating standing waves. Positions of anti-nodes (or nodes) of respective standing waves of the lower limit mode and the upper limit mode are thus shifted from each other.
Such a surface acoustic wave device disclosed in the first example of related art can surely exhibit an excellent frequency temperature characteristic, be suitable for increasing frequencies, and reduce a variation amount of the frequencies caused by a variation of the film thickness. However, in the first example of related art, manufacturing errors in mass production is not considered. For example, in a manufacturing process of a SAW device, when a resist pattern is formed and an electrode pattern is formed by wet etching, a difference in the thickness or the width of the resist pattern, and influence of side etching proceeding etching from side surfaces of the electrode pattern may cause errors in a line width of electrode fingers constituting an IDT. When the electrode pattern is formed by dry etching, a variation in the line width of the electrode fingers due to the side etching is reduced. However, a variation in the line width of the electrode fingers caused by a variation of the thickness and the width of the resist pattern may occur similarly to the case of wet etching.
Further, in a surface acoustic wave device made of a quartz crystal substrate with an in-plane rotated angle as disclosed in the first example of related art, when a line metalization ratio η between individual products varies due to manufacturing errors or the like, a variation amount of frequencies when a temperature is changed may largely change. That is, a variation in a frequency temperature characteristic becomes large. This becomes a major issue in reliability and quality of products in mass production in which a variation of line widths occurs to no small extent.